All Digital Subscriber Line (DSL) techniques are collectively referred to as the xDSL, which is a technique for high speed data transmission over a telephone twisted pair. In addition to the base band transmission DSL based upon the Integrated Services Digital Network (ISDN) and the like, the base band transmission xDSL makes use of the frequency division multiplexing technique to make the xDSL and the Plain Old Telephone Service (POTS) coexist on the same twisted pair, where the xDSL occupies the high frequency band and the POTS occupies the base band part below 4 KHz. A system providing multiple accesses for xDSL signals may be referred to as a DSL Access Multiplexer (DSLAM).
As a transmission channel, the telephone twisted pair has a distortion-free information capacity which shall satisfy the Shannon channel capacity formula:
  C  =      B    ·                  log        2            ⁡              (                  1          +                      S            N                          )            
In the above formula, C denotes a channel capacity, B denotes a channel bandwidth, S denotes signal energy, and N denotes noise energy. It can be seen that an increase of the channel bandwidth and the signal energy can increase the transmission capacity of the channel. However, the channel bandwidth is dependent upon an amplitude-frequency characteristic of the channel, and the signal energy is defined by devices, frequency spectrum compatibility, etc., so that both of them are defined within a certain range. Consequently, the transmission capacity of the channel can not be further increased in the case of the two defined conditions. From another point of view, the transmission capacity of the channel can be increased appropriately if the noise energy is reduced.
The increasing frequency band used for the xDSL technique exacerbates the crosstalk, especially the crosstalk at a high frequency band. Because the xDSL adopts frequency division multiplexing for uplink and downlink channels, a near-end crosstalk may not influence the system performance considerably, but a far-end crosstalk may influence seriously the transmission performance of lines. When an xDSL service is required to be enabled for multiple branches of users over a bundle of cables, some lines may have a low speed, unstable performance and even can not be enabled due to the far-end crosstalk, which may eventually result in a low line activation ratio of the DSLAM.
FIG. 1 illustrates a schematic diagram of a far-end crosstalk, where x1, x2 and x3 denote signal transmitting points, y1, y2 and y3 denote corresponding far-end signal receiving points, solid line arrows denote normal signal transmission, and dotted line arrows denote a crosstalk caused by a signal transmitting point to the receiving points corresponding to other signal transmitting points. As apparent from FIG. 1, transmitting signals of the points x2 and x3 are crosstalk sources for the transmitting signals of the point x1, and naturally transmitting signals of the point x1 are crosstalk sources for the signals of the points x2 and x3. Therefore, for clarity, a branch of transmitting signals is described as a reference object while regarding other signals as their crosstalk sources hereinafter. Such descriptions can be adaptive to respective branches of signals. Distinguishing names used for signals are merely for convenience, but not intended to differentiate the signals substantively.
The vectored-DSL technique has been currently proposed in the industry. The vectored-DSL technique mainly adopts a signal processing method to cancel out a far-end crosstalk among respective branches of signals by use of the coordinated transmission and reception feature at the DSLAM end. FIG. 2 illustrates a schematic diagram of transmission of signals from multiple end users respectively over a shared channel and coordinated reception at the DSLAM end. The far-end crosstalk may be cancelled primarily in a fixed filter manner in the prior art, and a general flow includes the following steps.
S1. Y(f) is transformed by Y{circumflex over (()}f)=Q*Y(f);
Y(f)=[Y1(f) Y2(f) . . . YL(f)]T, where Yi(f), (i=1,2, . . . L) denotes a channel output signal corresponding to a signal Xi(f), and Y(f)=H(f)X(f)+N(f);
H(f) denotes a channel transmission matrix in the frequency domain, in which a principal diagonal element denotes a line transmission function, and an off-diagonal element at the kth row and the mth column denotes a frequency domain crosstalk coefficient of the mth line to the kth line in a transmission channel;
X(f)=[X1(f) X2(f) . . . XL(f)]T, where Xi(f), (i=1, 2, . . . L) denotes the ith branch of input signals;
N(f) denotes the noise in the channel; and
Q* denotes a conjugate transpose matrix of a unitary matrix resulted from orthogonal triangular decomposition of H(f).
S2. Channel input signals are estimated in a general decision feedback equalization method according to the Y{circumflex over (()}f) obtained in step S1 and a pre-obtained H(f). Step S1 includes the following steps.
A. An initial value of the variable i is set as L;
B. Noise Ni is cancelled in a predetermined decision rule;
C. The input signal Xi is estimated from the formula:
                    X        i            ⁡              (        f        )              =                  (                                                            Y                ^                            i                        ⁡                          (              f              )                                -                                    ∑                              j                =                                  i                  +                  1                                            L                        ⁢                                          R                                  i                  ,                  j                                            ⁢                                                X                  j                                ⁡                                  (                  f                  )                                                                    )            /              R                  i          ,          i                      ;
where R denotes an upper triangular matrix resulting from orthogonal triangular decomposition of H(f).
D. If i>1, i is assigned with the value of i−1, and the flow returns to step S2 until i=1 where the flow ends.
As apparent from the implementation of the method, the channel transmission matrix should be known in advance to cancel out the far-end crosstalk. However, the matrix may be difficult to be obtained accurately and conveniently, and the matrix per se has a feature of slowly time-varying and may be susceptible to a transmission environmental factor. Consequently, the above solution may be difficult to implement in practice.